The present invention relates to a plasma spraying method and apparatus for obtaining a sprayed film having abrasion resistance and corrosion resistance or a film of a ceramic solid electrolyte or the like required to have high functionality, and a sprayed film obtained by the method.
In this type of previous invention, as shown in FIG. 4, a granular spraying material 56 is supplied to a portion close to an anode 53 of a plasma torch 51 through a material supply nozzle 55 to form high-temperature fine-grain melt droplets 57 which are conveyed and accelerated by a plasma flame 54 discharged from an outlet 58 of the plasma torch 51 and which are caused to collide with a substrate 59 placed in front of the plasma flame 54 to form a sprayed film 60 of the spraying material on a surface of the substrate.
In this case, the plasma flame 54 is enlarged to the shape shown in the drawing due to the attraction of surrounding air 61 in the space ranging from the outlet of the plasma torch 51 to the substrate 59. The heat history of the melt droplets 57 in the plasma flame 54 is widely changed by the course. This deteriorates the uniformity of the sprayed film 60 formed on the surface of the substrate 59 and decreases the density thereof.
In order to solve the above problem, a plasma spraying apparatus as shown in FIG. 5 is proposed in which the length from the tip 62 of a cathode 52 of a plasma torch 51 to the anode point 63 of an anode 53 is longer than that shown in FIG. 4, and an annular gas passage 65 is concentrically provided outside a plasma gas passage 64 provided around the cathode 52. Tangential passages 67 are also formed in an annular ring wall 66 between both gas passages 64, 65 so that plasma gas 69 introduced from a plasma gas inlet 68 is caused to flow to the outlet 58 while being circulated by the plasma gas passage 64 around the cathode 52. The gas is thus sufficiently heated by a relatively long arc 50 produced between the cathode tip 62 and the anode point 63 to form an elongated plasma flame 54, whereby the focusing and stability of a beam of the melt droplets 57 contained in the flame 54 can be improved.
However, if the length L6 of the plasma flame 54 is sufficiently longer than the length L5 of the plasma flame 54 shown in FIG. 4, as described above, when the droplets of the spraying material collide with the substrate 59, there is the danger that the plasma flame 54 for conveying the droplets also collides with the substrate 59 and thus damages it due to overheating.
In order to prevent the substrate from being damaged by the elongated plasma flame 54, it is thought that the substrate 59 is disposed far away from the outlet 58 of the plasma torch 51 in the plasma flame 54 so that the temperature of the plasma flame 54 is decreased by air 61 in the space between the outlet 58 of the plasma torch 51 and the substrate 59.
However, this method decreases the speed of the droplets 57 at the time of collision with the substrate 59 and decreases the denseness and adhesion of the sprayed film 60 formed by cooling the melt droplets 57. Namely, conditions of the length L5, L6 which are required for preventing the damage of the substrate 59 and for improving the quality of the sprayed film 60 contradict each other. It is difficult to satisfy the both conditions.